Corrosion preventive process



United States Patent 3,128,210 CORROSION PREVENTIVE PROCESS Lewis J. Brown, North Wales, Pa., assignor to Pennsalt Chemicals Corporation, a corporation of Pennsylvama No Drawing. Filed Nov. 23, 1956, Ser. No. 623,810 2 Claims. (Cl. 134-29) This invention relates to the prevention of corrosion of metal surfaces, and more particularly to the prevention of a specific defect of steel surfaces known as pinpoint rust.

Steel plates, sheets or other formed articles, after mechanical forming, are usually cleaned in aqueous alkali for the purpose of removing the accumulated shop dirt, metal dust and lubricant residues, etc. picked up in the forming process. The steel, after cleaning but prior to annealing and final surface finishing (i.e., tin plating, galvanizing, painting, etc.), is subject to a particularly troublesome form of corrosion which consists in very small, relatively deeply pitted rust spots. These may appear on exposure to normally humid atmospheres after cleaning but prior to annealing, or else they develop during annealing. Known as pinpoint rust, this form of corrosion can be very costly to the steel manufacturers in terms of rejected steel stock, since the steel surface is eifectively destroyed thereby and cannot be restored by any known method. Prior to the present invention, despite long and diligent searching, no means of either prevention or alleviation of this problem had been found.

It has now been discovered that pinpoint rust can be greatly reduced or entirely prevented by the addition of one surprisingly simple process step in the normal cleaning cycle that takes place prior to annealing, this step consisting in subjecting the metal to a final rinse in water that is substantially free from a certain combination of contaminants, as hereinafter more fully described.

The invention is based on the discovery that the essential causative factor in the formation of pinpoint rust is the presence in the final rinse water of the combination of (1) an alkali and (2) any or all of the anions of the group consisting of chlorides, sulfates and nitrates. Surprisingly, it has been found that only the combination causes pinpoint rust. If one or the other component of the combination is removed from the rinse water, no pinpoint rust will occur. Moreover, if the amount of one or the other component is reduced, the amount and severity of the pinpoint rust is also proportionately reduced. In order to prevent entirely the formation of pinpoint rust, it has been found that the concentration of at least one component must be less than about 5 parts per million (p.p.m.). A reduction in pinpoint rusting to a degree that may be adequate in most cases can be obtained if this concentration is less than about ppm.

It is apparent that ordinarily, unless deliberate steps are taken to remove at least one of these contaminants the offending combination will be present in rinse water, the alkali being introduced normally as entrained cleaning solution on the steel itself, and the anions coming in as hardness (e.g., CaCl MgSO etc.) universally present in industrial waters. Indeed most river and well waters generally contain not only suflicient of the anions but also enough alkali, in the form of dissolved bicarbonate, to cause pinpoint rust.

In practicing the present invention, the important point to note is that after steel has been cleaned in alkali the final treatment of the steel surface prior to annealing should consist of an aqueous rinse that is essentially free from the harmful combination. The protection from pinpoint rust afforded by this final rinse is essentially independent of the preceding cleaning operations, the details of which vary widely in different steel mills. When the ice steel is given this final rinsing, the development of the destructive, deeply pitted type of rust on subsequent annealing is greatly reduced or completely prevented; any rust that does develop is of the superficial, disseminated type that is largely dissipated during annealing or easily removed by acid pickling.

Several practical ways in which to insure that the final rinse is free of either the alkali, the named anions, or both, are readily available, and the choice between them will naturally be dictated by the conditions present in the particular steel mill. Thus, in some mills it may be convenient and economical to use steam condensate as the source of Water for the final rinse, while other mills may prefer to deionize the water with ion exchange resins. The manner of application of the rinse Water is not limited to any particular method, provided the steel surface is thoroughly rinsed; immersion, spray, etc. are equally suitable. While it is preferred that the final rinse 'water be essentially clean, nevertheless minor amounts of foreign matter such as smut, organic matter, etc., need not be too scrupulously eliminated, since these substances have been shown to play no active roll in the formation of pinpoint rust.

The following table serves to illustrate the effect of the final rinse on the subsequent development of rust. The steel used in the comparative tests in this table was black plate (i.e., cold-rolled sheet steel, used as tinning stock), from several mills. After the final rinsing treatment indicated in the table, the variously treated samples were exposed to 92% humidity for about a day, in order to develop fully any tendency they might have to rust.

Table 1 Pinpoint Anion Alkali Rust 1 Chloride N 3.4Sl04 ooooco NaOL- NaNOa NaNOs NaaSO4. Na SO4 Nalsiollivl N a4SiO4 1 Legend: 0=n0ne, 1+=slight, 2+=modcrate, 3+=severe. 2 Superficial, non-pitting, streak rust present. This rust; dissipated when the stock was subsequently annealed.

It will be noted from Table I that the type of alkali present is immaterial; strong bases such as caustic soda and sodium silicate give the same results as weak bases such as the carbonates, bicarbonates, and phosphates in that alone in the rinse water they do not cause pinpoint rusting but in combination with chloride, sulfate or nitrate severe pitting results.

The following examples illustrate the use of ion exchange resins in preventing pinpoint rust on black plate. In these examples the procedure followed, briefly, was:

(1) Electrolytic cleaning in commercial sodium orthosilicate,

(2) Brushing and rinsing to remove smut (tap water),

(3) Final rinsing,

(4) Air blast drying, and

(5) Either (a) 91% relative humidity test at room temperature or (b) stacking and box annealing.

Example 1 Black plate after alkaline cleaning was rinsed in water having a total hardness of 75 ppm. This plate subsequently developed severe pinpoint rust. The same water was passed through a two-bed ion exchange system consisting of, first, Amberlite IR-l20 (a cross-linked polystyrene copolymer cation exchange resin, sold by Rohm & Haas Co.) and second, Amberlite IR-45 (a weakly basic anion exchange resin, also a Rohm & Haas product). Black plate given a final rinse in this deionized water remained free from rust on subsequent exposure to humidity.

Example 11 Hard water (75 p.p.rn.) was passed through a strongly basic alkanol amine anion exchange resin (Amberlite IRA-410, Rohm & Haas). The effluent, used as the final rinse following alkaline cleaning of black plate, Was efiective in preventing pinpoint rust.

The present invention is not to be regarded as limited to the particular methods illustrated by which the harmful combination is removed in order to provide a rustpreventive rinse, as it will be apparent that many alternative methods of achieving this result are available. This invention is intended to include these alternatives as well.

Having described my invention, I claim:

1. In the manufacture of steel wherein the steel surface is cleaned in aqueous alkali,.rinsed with water containing substantially in excess of 10 parts per million of anions of the group consisting of chlorides, sulfates and nitrates and then annealed, the method of reducing pin-point rusting of the steel surface which comprises subjecting said steel surface prior to annealing, and subsequent to such rinsing in said water containing substantially in excess of 10 parts per million of the said anions, to a final rinse with Water which contains less than 10 parts per million of said anions and then annealing said steel.

2. A method in accordance with claim 1 in which said final rinse water contains less than 5 parts per million of said anions.

References Cited in the file of this patent UNITED STATES PATENTS 1,989,884 Reinhardt Feb. 5, 1935 2,259,260 Matteson et al. Oct. 14, 194-1 2,338,689 Parker et al. Ian. 4, 1944 2,561,158 Tuttle July 17, 1951 2,633,437 Detjen Mar. 31, 1953 2,661,314 Criddle Dec. 1, 1953 

1. IN THE MANUFACTURE OF STEEL WHEREIN THE STEEL SURFACE IS CLEANED IN AQUEOUS ALKALI, RINSED WITH WATER CONTAINING SUBSTANTIALLY IN EXCESS OF 10 PARTS PER MILLION OF ANIONS OF THE GROUP CONSISTING OF CHLORIDES, SULFATES AND NITRATES AND THEN ANNEALED, THE METHOD OF REDUCING PIN-POINT RUSTING OF THE STEEL SURFACE WHICH COMPRISES SUBJECTING SAID STEEL SURFACE PRIOR TO ANNEALING, AND SUBSEQUENT TO SUCH RINSING IN SAID WATER CONTAINING SUBSTANTIALLY IN EXCESS OF 10 PARTS PER MILLION OF THE SAID ANIONS, TO A FINAL RINSE WITH WATER WHICH CONTAINS LESS THAN 10 PARTS PER MILLION OF SAID ANIONS AND THEN ANNEALING SAID STEEL. 